1) Field of the Invention
The present invention relates to a thermal fixing unit in an image forming apparatus such as a copying machine, a printer, and a facsimile, and particularly to a fixing unit in the form of a belt.
2) Description of the Related Art
There is a strong demand that the image forming apparatuses operate at high-speed and also consume less energy. One approach to achieve this is to improve the thermal efficiency of the fixing unit of the image forming apparatuses. The examples of the image forming apparatus are copying machines, printers, facsimile machines, and multifunction peripherals.
In the image forming apparatuses, a toner image is formed on a recording medium generally by two methods: an image transfer method and a direct transfer method. The examples of the recording medium are an ordinary paper, a photosensitive paper, an electrostatic recording paper, and an OHP (overhead projector) sheet. The toner image is then fixed to the recording medium by a fixing unit. The fixing units mainly employ two methods to fix the toner image: a contact heating process and an electromagnetic induction heating process. The examples of the contact heating process are a heat roller process and a film heating process.
The fixing unit that uses the heat roller process, basically includes a pair of rotating rollers: a fixing roller and a pressurizing roller. The fixing roller is heated by a heat source such as a halogen lamp. The pressurizing roller is in contact with the fixing roller and also applies pressure to the fixing roller. A paper with the toner image is introduced between the so called fixing nip, which is a contact portion of the pair of the rotating rollers, and then carried forward. The toner image is fused or fixed to the paper due to heat and pressure while the paper passes between the fixing roller and the pressurizing roller.
A conventional fixing unit that uses the film heating process has been proposed in Japanese Patent Application Laid-open Publication Nos. S63-313182 and H1-263679. The fixing unit that uses the film heating process includes bringing the paper near a heating member while there is a heat-resistant, thin, fixing film between them, and allowing the paper to slide over the fixing film. The heating member can be a ceramic heater that includes a resistive layer on a ceramic substrate such as alumina and aluminum nitride that has properties such as high heat resistivity, insulation, and good thermal conductivity. Because the fixing film is thin and has a low thermal capacity, an efficiency of heat transfer is higher than that of the fixing unit that uses the heat roller process. Therefore, the warm-up time of the fixing unit that uses the film heating process is shorter. This allows a quicker start-up and energy conservation.
Japanese Patent Application Laid-open Publication No. H8-22206 discloses a conventional fixing machine that uses the electromagnetic induction heating process. This fixing machine includes a magnetic metal member and a heating member. An alternating magnetic field is applied to the magnetic metal member. As a result, eddy currents are generated in the magnetic metal member and Joule's heat is produced in the magnetic metal member. The metal member is heated by the Joule's heat.
Various toners are used in the fixing unit that uses the electromagnetic induction heating process. For example, Japanese Patent Application Laid-open Publication No. H11-344830 proposes a toner that includes a strong magnetic material and a resin that has a glass transition point in a range of 45° C. to 65° C. and a softening point in a range of 80° C. to 140° C.
Moreover, Japanese Patent Application Laid-open Publication No. 2001-235893 proposes a toner that includes a styrene-acrylic resin in which the glass transition point and an MI value are regulated, and a fixed polyolefin wax are used.
Furthermore, Japanese Patent Application Laid-open Publication No.2001-272812 proposes a toner that includes a polyester resin that has a regulated endothermic peak of wax and a regulated dynamic viscoelasticity is used.
Moreover, Japanese Patent Application Laid-open Publication No. 2001-272818 proposes a toner of which a molecular weight distribution and an endothermic peak temperature are regulated.
Furthermore, Japanese Patent Application Laid-open Publication No. 2002-91075 proposes a toner that includes a resin in which a composition of both acid and alcohol are regulated.
Moreover, Japanese Patent Application Laid-open Publication No. 2002-91076 discloses a toner of which the melt viscosity is regulated and includes a polyester resin in which an insoluble content of THF (tetrahydrofuran) and an acid value are regulated.
Moreover, a technology in which a low temperature offset and a high temperature offset are prevented by regulating a heat capacity of a belt, has been disclosed in Japanese Patent No. 2813297.
Furthermore, a technology in which a separation is improved by improving a permeating property of a mold releasing agent by using a spherical shaped toner that contains a mold releasing agent that has a low melting point, in a heating passage before the nip of the belt fixing unit, has been disclosed in Japanese Patent No. 3423616.